Absorbent embryonic webs are a staple of everyday life. Absorbent embryonic webs include cellulosic fibrous structures, absorbent foams, etc. Cellulosic fibrous structures have become a staple of everyday life. Cellulosic fibrous structures are found in facial tissues, toilet tissues and paper toweling.
In the manufacture of cellulosic fibrous structures, a wet embryonic web of cellulosic fibers dispersed in a liquid carrier is deposited onto a forming wire. The wet embryonic web may be dried by any one of or combinations of several known means, each of which drying means will affect the properties of the resulting cellulosic fibrous structure. For example, the drying means and process can influence the softness, caliper, tensile strength, and absorbency of the resulting cellulosic fibrous structure. Also the means and process used to dry the cellulosic fibrous structure affects the rate at which it can be manufactured, without being rate limited by such drying means and process.
An example of one drying means is felt belts. Felt drying belts have long been used to dewater an embryonic cellulosic fibrous structure through capillary flow of the liquid carrier into a permeable felt medium held in contact with the embryonic web. However, dewatering a cellulosic fibrous structure into and by using a felt belt results in overall uniform compression and compaction of the embryonic cellulosic fibrous structure web to be dried.
Felt belt drying may be assisted by a vacuum, or may be assisted by opposed press rolls. The press rolls maximize the mechanical compression of the felt against the cellulosic fibrous structure. Examples of felt belt drying are illustrated in U.S. Pat. No. 4,329,201 issued May 11, 1982 to Bolton and U.S. Pat. No. 4,888,096 issued Dec. 19, 1989 to Cowan et al.
Drying cellulosic fibrous structures through vacuum dewatering, without the aid of felt belts is known in the art. Vacuum dewatering of the cellulosic fibrous structure mechanically removes moisture from the cellulosic fibrous structure while the moisture is in the liquid form. Furthermore, the vacuum deflects discrete regions of the cellulosic fibrous structure into the deflection conduits of the drying belts and strongly contributes to having different amounts of moisture in the various regions of the cellulosic fibrous structure. Similarly, drying a cellulosic fibrous structure through a vacuum assisted capillary flow, using a porous cylinder having preferential pore sizes is known in the art as well. Examples of such vacuum driven drying techniques are illustrated in commonly assigned U.S. Pat. No. 4,556,450 issued Dec. 3, 1985 to Chuang et al. and U.S. Pat. No. 4,973,385 issued Nov. 27, 1990 to Jean et al.
In yet another drying process, considerable success has been achieved drying the embryonic web of a cellulosic fibrous structure by through-air drying. In a typical through-air drying process, a foraminous air permeable belt supports the embryonic web to be dried. Hot air flow passes through the cellulosic fibrous structure, then through the permeable belt or vice versa. The air flow principally dries the embryonic web by evaporation. Regions coincident with and deflected into the foramina in the air permeable belt are preferentially dried and the caliper of the resulting cellulosic fibrous structure increased. Regions coincident the knuckles in the air permeable belt are dried to a lesser extent.
Several improvements to the air permeable belts used in through-air drying have been accomplished in the art. For example, the air permeable belt may be made with a high open area (at least forty percent). Or, the belt may be made to have reduced air permeability. Reduced air permeability may be accomplished by applying a resinous mixture to obturate the interstices between woven yarns in the belt. The drying belt may be impregnated with metallic particles to increase its thermal conductivity and reduce its emissivity or, alternatively, the drying belt may be constructed from a photosensitive resin comprising a continuous network. The drying belt may be specially adapted for high temperature airflows, of up to about 815 degrees C. (1500 degrees F.). Examples of such through-air drying technology are found in U.S. Patent Re. 28,459 reissued Jul. 1, 1975 to Cole et al.; U.S. Pat. No. 4,172,910 issued Oct. 30, 1979 to Rotar; U.S. Pat. No. 4,251,928 issued Feb. 24, 1981 to Rotar et al.; commonly assigned U.S. Pat. No. 4,528,239 issued Jul. 9, 1985 to Trokhan; and U.S. Pat. No. 4,921,750 issued May 1, 1990 to Todd. Additionally, several attempts have been made in the art to regulate the drying profile of the cellulosic fibrous structure while it is still an embryonic web to be dried. Such attempts may use either the drying belt, or an infrared dryer in combination with a Yankee hood. Examples of profiled drying are illustrated in U.S. Pat. No. 4,583,302 issued Apr. 22, 1986 to Smith and U.S. Pat. No. 4,942,675 issued Jul. 24, 1990 to Sundovist.
The foregoing art, even that specifically addressed to through-air drying, does not address the problems encountered when drying a multi-region cellulosic fibrous structure. For example, a first region of the cellulosic fibrous structure, having a lesser absolute moisture, density or basis weight than a second region, will typically have relatively greater airflow therethrough than the second region. This relatively greater airflow occurs because the first region of lesser absolute moisture, density or basis weight presents a proportionately lesser flow resistance to the air passing through such region.
This problem is exacerbated when the multi-region cellulosic fibrous structure to be dried is transferred to a Yankee drying drum. On a Yankee drying drum, isolated discrete regions of the cellulosic fibrous structure are in intimate contact with the circumference of a heated cylinder and hot air from a hood is introduced to the surface of the cellulosic fibrous structure opposite the heated cylinder. However, typically the most intimate contact with the Yankee drying drum occurs at the high density or high basis weight regions, which are not as dry as the low density or low basis weight regions. Preferential drying of the low density regions occurs by convective transfer of the heat from the airflow in the Yankee drying drum hood. Accordingly, the production rate of the cellulosic fibrous structure must be slowed, to compensate for the greater moisture in the high density or high basis weight region. To allow complete drying of the high density and high basis weight regions of the cellulosic fibrous structure to occur and to prevent scorching or burning of the already dried low density or low basis weight regions by the air from the hood, the Yankee hood air temperature must be decreased and the residence time of the cellulosic fibrous structure in the Yankee hood must be increased, slowing the production rate.
Another drawback to the approaches in the prior art (except those that use mechanical compression, such as felt belts) is that each relies upon supporting the cellulosic fibrous structure to be dried. Airflow is directed towards the cellulosic fibrous structure and is transferred through the supporting belt, or, alternatively, flows through the drying belt to the cellulosic fibrous structure. Differences in flow resistance through the belt or through the cellulosic fibrous structure, amplify differences in moisture distribution within the cellulosic fibrous structure, and/or creates differences in moisture distribution where none previously existed. However, no attempt has been made in the art to tailor the airflow to the differences in various regions of the cellulosic fibrous structure.
One improvement in the art which addresses this problem is illustrated by commonly assigned U.S. Pat. No. 5,274,930 issued Jan. 4, 1994 to Ensign et al. and disclosing limiting orifice drying of cellulosic fibrous structures in conjunction with through-air drying, which patent is incorporated herein by reference. This patent teaches an apparatus utilizing a micropore drying medium which has a greater flow resistance than the interstices between the fibers of the cellulosic fibrous structure. The micropore medium is therefore the limiting orifice in the through-air drying process so that an equal, or at best a more uniform, moisture distribution is achieved in the drying process.
The limiting orifice through-air-drying apparatus of the Ensign et al. patent teaches having one or more zones with either a subatmospheric pressure or a positive pressure to promote airflow in either direction.
However, this patent (8:17-26) also teaches that as the basis weight of the embryonic web increased, greater residence time on the micropore medium would be necessary, as logic would dictate. Specifically, it taught a common tissue paper basis weight (12 pounds per 3,000 square feet) would require a residence time of at least about 250 milliseconds on the micropore medium.
Applicants have unexpectedly found that the necessary residence time in the first zone can be reduced, providing the limiting orifice through-air drying apparatus is divided into plural zones. Furthermore, it has unexpectedly been found that the overall energy consumption of the apparatus can be reduced utilizing proper zones. Specifically, less fan horsepower is required if the zones are properly sized and selected. Fan horsepower reductions of up to 10 to 15 percent over the original apparatus disclosed in the aforementioned Ensign et al. patent can be by utilizing the present invention. At an advertised annual operating cost of $200 to $250 per horsepower per year the potential savings can be significant. Accordingly, it is an object of this invention to provide a limiting orifice through-air drying apparatus having a micropore medium which can be used in conjunction with through-air drying to produce cellulosic fibrous structures. It is, furthermore, an object of this invention to provide a limiting orifice through-air drying apparatus which reduces the necessary residence time and requires less energy than had previously been thought in the prior art.